This invention relates to improvements of a synchronous belt and improvements of a method of producing the synchronous belt, and particularly relates to improvements of a cord of a synchronous belt having no facing fabric.
In power transmission belts used for office automation (OA) gears, especially in synchronous belts such as a miniature synchronous belt, there have been used for teeth two types of materials, i.e., vulcanized rubber and casting urethane elastmer. Out of these two types of materials, the latter is generally used with a cord made of aramid fibers which serves as a tension member (See Japanese Patent Application Laid-Open Gazette No.5-44181), because the aramid fiber is excellent in properties such as strength, elasticity and heat resistance. Otherwise, a cord composed of a metallic wire would be susceptible to rust and possess excessive stiffness. On the contrary, since a cord made of polyester fibers has low stiffness (small modulus of elasticity), this is unsuitable for heavy duty power transmission.
However, the synchronous belt using a cord made of aramid fibers has a problem in its use for household electrical appliances and OA gears in which the center distance between pulleys is fixed, because the cord is increased about 0.20% in length due to the absorption of moisture.
Meanwhile, a cord made of glass fibers has been also used in many rubber-made synchronous belts because of its excellent dimensional stability (See Japanese Patent Application Laid-Open Gazette No.63-76935).
In this view, it can be considered to form a cord from inorganic fibers such as glass fiber having excellent dimensional stability and use it in a synchronous belt made of casting urethane elastomer. However, such a cord formed of glass fibers have been seldom used in the synchronous belts made of casting urethane elastomer.
It can be assumed that (a) one of the reasons is that the cord is susceptible to damage from a mold at the molding and that (b) the other reason is that inter-filament parts of the cord are impregnated with casting urethane elastomer thereby rising stiffness. These two reasons (a) and (b) will discuss below in detail.
First, discussion is made about the reason (a). Since the teeth of the synchronous belt made of casting urethane elastomer cannot be coated with a facing fabric, the cord of the synchronous belt is damaged.
If a synchronous belt with a facing fabric is produced by the use of casting urethane elastomer, there may be employed the manner that rolls a facing fabric around an inner mold which has at the periphery a teeth-like forming surface for forming a bottom face of the synchronous belt, winds a cord around the facing fabric, sets outside the inner mold a cylindrical outer mold having at the inner periphery a forming surface for forming a back face of the synchronous belt and pours urethane elastomer into a space between the facing fabric and the outer mold. However, since the urethane elastomer has a low viscosity to pass through textures of the facing fabric, it is impossible to press the facing fabric against the teeth-like forming surface of the inner mold. Therefore, whereas most of rubber-made synchronous belts each have teeth coated with a facing fabric, there can be obtained no synchronous belt made of urethane elastomer which has teeth coated with a facing fabric.
Therefore, the synchronous belts are produced with no facing fabric provided, so that, in many cases, the portions of the cord located at bottom lands are not coated with urethane elastomer to be exposed. Meanwhile, on the teeth-like forming surface of the inner mold, projections are formed for preventing the displacement of the cord. When the molded synchronous belt is removed from the inner mold, the cord may be damaged by the projections of the inner mold.
Next, discussion is made about the reason (b). In the formation of the synchronous belt made of urethane elastomer, urethane elastomer is generally poured under a reduced pressure for ease of the pouring and for degassing, so that casting urethane elastomer is entered to inter-filament parts of the cord thereby extremely hardening the cord. Such hardened cord is readily broken and readily generates heat at the belt run.